A process for the preparation of highly pure, elemental silicon and other transition metals is disclosed in U.S. Pat. No. 4,442,082 which may be practiced by the reduction of a silicon tetrahalide using an alkali metal. That patent is incorporated by reference. High purity silicon is useful for the direct production of electricity from solar energy and for other purposes.
Briefly, the process for the preparation of silicon is based on the reduction of a silicon tetrahalide such as silicon tetrafluoride in the vaporphase by the use of an elemental form of an alkali metal such as sodium which produces a mixture of an alkali metal fluoride and elemental silicon.
After the reaction is complete aqueous leaching of the reaction mixture is usually employed to separate the alkali metal fluoride. Thereafter, melt consolidation may be used to prepare a polycrystalline silicon material that is suitable for use in Czochralski crystal pulling procedures.
The present invention is concerned with the discovery of the use of a critical range of a partial vacuum for the melt consolidation of a polycrystalline silicon powder. The use of a partial vacuum results in the removal of any trace amounts of carbon as well as the removal of any silica that is formed from the oxidation of silicon. In addition, the use of a partial vacuum permits the removal of trace amounts of boron and sodium which may be detrimental to the use of silicon for solar generation of electricity.
U.S. Pat. No. 4,388,286 describes a method for the purification of silicon which is based on heating silicon sawdust in an argon atmosphere adding sodium fluoride and thereafter applying a high vacuum (10 torr) to remove carbon as gaseous carbon monoxide and silica as silicon monoxide. The product obtained by this process is disclosed as containing slag and having a high sodium content. The application of the high vacuum causes the melted silicon to bubble and splatter from the crucible resulting in product losses. Accordingly, it is a primary object of this invention to provide an improved process for the melt consolidation of powdered silicon in a highly pure form.
It is also an object of this invention to provide a process for removing small amounts of carbon, boron and sodium from elemental silicon powder in a melt consolidation process.